The present invention relates generally to multiple element optical amplifier arrays used to achieve a high power beam and in particular, to a coherent beam combining system to facilitate such laser amplifier arrays.
The intensity and, hence, the power available from a single-mode optical fiber is limited by either optical surface damage or nonlinear optical effects. These limitations can be overcome by coherent beam-combining of the output power from multiple optical fibers. Fine control of the optical phase is required for any multi-fiber approach. In a master oscillator power amplifier (MOPA) configuration, the optical paths of each of the fibers have to be controlled to a fraction of the wavelength in order to coherently combine the individual outputs into a single, high-power beam. As a result of time varying thermal loads and other disturbances, an active feedback system is required to provide for both coherent addition and rapid slewing of the final beam direction
There have been a number of experimental and theoretical research efforts directed toward developing a practical scheme for coherent beam combining. A number of researchers have implemented electronic phase locking that has demonstrated high fringe visibility for both passive (R. R. Rice et, J. A. Davis, J. S. Whitely, J. H. Hollister, and N. F. Ruggieri, “Coherent Fiber MOPA,” 14th Annual Solid State and Diode Laser Technology Review, Albuquerque, N. Mex., May, 2001 and J. Abderegg, S. J. Brosnan, M. E. Weber, H. Komine, and M. G. Wickham, “8-Watt Coherently-Phased 4-Element Fiber Array,” Proceedings of the SPIE vol. 4974, pp. 1–6, 2003) and active systems (S. J. Augst, T. Y. Fan, and Antonio Sanchez, “Coherent Beam Combining and Phase Noise Measurements of Yb fiber Amplifiers,” Optics Letters, Vol. 29, No. 5, pp. 474–476, Mar. 1, 2004). In previously described electronic phase locked fiber arrays, each leg of the array is modulated at the same RF frequency or alternatively the reference beam is the only beam modulated at an RF frequency. The light emerging from each leg is then interfered with the light from a reference leg. Because the same RF frequency is used to modulate each array leg, the light from each leg must be sent to spatially separate photodetectors. Good fringe visibilities of greater than 94 percent, and hence, very low phase errors were measured. However, Abderegg et. al. reported that the spatial alignment had stringent requirements even when the fiber-to-fiber spacing was 3-mm. For practical purposes, any array locking method must confine most of the array power into a single lobe. This in turn requires the use of a closely packed array. The closer the array elements are in a system using multiple photodetectors, the more stringent the spatial alignment tolerances required to ensure that there is no interference from adjacent array elements.
All of the above systems required one photodetector per array element leading to a more complex system and requiring great care be taken to ensure that the light from adjacent array elements is eliminated from the photodetector. To achieve the required spatial isolation a heavy optical platform is needed. The external reference beam also adds to the optical complexity and increases the size and weight of the optical platform. Clearly it would be advantageous to have a coherent beam combining system that used a single photodetector and needed no external reference beam.
A system for electronic phase locking of optical arrays that eliminates the external reference beam and in addition uses only a single photodetector is the subject of a pending patent application and is hereby incorporated by reference (T. M. Shay, “Self-referenced Locking of Optical Coherence by Single-detector Electronic-frequency Tagging,” application Ser. No. 11/215,055). Self-referenced Locking of Optical Coherence by Single-detector Electronic-frequency Tagging eliminated drawbacks of the previous electronic phase locking systems while preserving high locking fidelity and scalability of electronic phase locking systems. It would be advantageous if the reference beam itself could be eliminated.